The invention relates to a method for reducing the NO.sub.x emission from a kiln plant where low volatile fuels are used for heat treatment of raw materials, in which kiln plant fuel can be fired in at least three different zones. In one of these at least three zones an amount c of fuel is burned, in a second of these zones an amount b of fuel is burned and to these second zone the NO containing exhaust gases from the other at least two zones are also fed. In the rest of these at least three zones an amount a of fuel is burned and at least a part of the raw materials are fed to these zones together with an oxygen containing gas. The total amount of fuel, b+a, burned in the second and in the last zones is determined by the need for treatment of the raw materials and the amounts of fuel, b and a, burned in the second (2) and the last zones, are adjusted in upward and downward direction until a minimum NO content is achieved in the exhaust gases from the second zone.
Nitrogen oxides, NO.sub.x, are formed during combustion due to oxidation of nitrogen in the fuel and due to oxidation of nitrogen in the combustion air. In case the temperature in the combustion zone is less than 1200.degree. C., NO.sub.x is formed only on the basis of the nitrogen which is present in the fuel. This type is called fuel NO.sub.x. If the temperature rises to a level beyond 1200.degree. C., nitrogen oxides will also be formed on the basis of the combustion air. This type is called thermal NO.sub.x. Approximately 95% of the nitrogen oxides which are formed as fuel NO.sub.x and thermal NO.sub.x consist of nitrogen oxide, NO. In a system in which fuel containing nitrogen is burned, the following types of reactions can take place:
(1) N.sub.fuel +O.fwdarw.NO PA1 (2) N.sub.fuel +NO.fwdarw.N.sub.2 +O
Reaction (1) indicates that the NO formation in the zone will depend on the content of nitrogen in the fuel and on the oxygen content of the gases in that zone. Reaction (2) indicates that if NO is already present in the gas which is fed to this zone, the amount of NO present in the supplied gas will be reduced by means of the nitrogen compounds which are released from the fuel. The net production of NO thus also depends on the NO content in the supplied gas and as the reaction rate of reaction (2) rises more quickly with the temperature than the reaction rate for reaction (1), an increase of the temperature will in net terms lead to a reduction of the amount of NO.sub.x which is emitted from the calcining zone. In connection with high temperature combustion in the calciner it is known that if the temperature is increased by approximately 100.degree. C., it is possible to lower the NO.sub.x from the calciner by 10-15%. The upper limit of this advantage is 1200.degree. C., at this temperature the forming of thermal NO.sub.x from the combustion air will exceed the reduction of NO by reaction (2).
When the kiln plant is used for manufacturing cement clinker the heat treatment consists of preheating, calcining sintering and cooling of the mineral raw materials. The three zones where the N-containing fuel is burned are in the sintering zone, in a kiln, and at two locations in the calcining zone, in a calciner and in at least one burning compartment. According to the description of this invention a `burning compartment` is a zone where fuel is fired and where at the same time materials to be treated are added. A `calciner` is a burning compartment located in the kiln gas duct where the exhaust gases from the kiln will pass through it.
The temperature in a kiln plant for cement clinker manufacture is only in excess of 1200.degree. C. in the sintering kiln itself. The necessary temperature and time of retention in the kiln depend on the characteristics of the raw materials. A raw material with poor burning characteristics will thus require a high temperature and/or prolonged time of retention. Such conditions with a high flame temperature up to 2000.degree. C. will increase the rate of NO.sub.x emission substantially.
Measurements have indicated that the content of volatiles in the used fuel and the temperature at which calcination is taking place are factors which influence the formation of NO.sub.x in the calcining zone. The higher the content of volatiles in the fuel, the smaller it appears is the amount of N.sub.fuel which is transformed into NO.sub.x.
It is a recognised fact that various advantages may be obtained by constructing a calcination zone equipped with an additional burning compartment which is located in such a way that the burning compartment is exclusively fed with tertiary air from the cooler. Such a burning compartment would be considered to be included in the calcination zone if raw materials were added to the compartment.
A plant of this type is described in European patent No. 103423 (F. L. Smidth & Co. A/S, corresponds to DK-C-151319). Here from is known a plant (SLC-S) for calcination of cement raw materials, in which it has been taken into account that it may be difficult to achieve a complete burn-out of the fuel which is used in the calciner. In this plant raw meal is fed after it has passed through la cyclone preheater (18, 18', 19, 20, 21), to a burning compartment (4) in which the raw material is calcined in hot air from the cooler (2). Subsequent to preheating, raw meal is directed to the calcining zone at two locations: in the burning compartment (4) and in the kiln gas duct (28) or the retention compartment (29). As indicated in claim 4 it is possible to feed fuel to the kiln gas duct (28) via a burner (45), but according to column 6, line 13-27, this supplementary fuel is supplied in order to ensure that the amount of raw meal in the kiln gas duct can be increased.
From U.S. Pat. No. 4,014,641 (Mitsubishi) is known a plant for calcination of cement raw materials in which the amount of nitrogen oxide in the exhaust gases of the kiln is reduced by generating an area in the kiln gas duct to which reducing gas is fed. Hot air from the cooler (via a duct (5)) and hot air from the kiln (via a duct (13)) are routed to a cyclone preheater (14, 15, 16, 17) in which raw material is preheated in counter-current to the hot gas from the cooler and from the kiln. In the area of the kiln gas duct which is located below the supply duct (5) from the cooler, reducing conditions are generated by introducing reducing gases via a duct (12). The reducing gases are formed in the calciner (8) as the air volume which is being fed to the calciner is sufficient to cause gasification of the fuel in the calciner, but insufficient to cause a complete burn-out of the fuel in the calciner (column 4, line 1-5). A particular disadvantage of this plant is that fuels that are difficult to ignite and slow burning, such as petroleum coke, anthracite and other coals with a low gas content cannot be used, as they would produce a large unburned coke residue which would be precipitated in the rotary kiln, and, as a consequence hereof, would be causing problems in terms of sulphur expulsion and caking.
From U.S. Pat. No. 5,364,265 (CLE) is known yet another calcining system in which the NO.sub.x emission is limited by formation of reducing gases, viz. CO and H.sub.2, in a burning compartment (20). The coke formed in the burning compartment during this process has quite distinctive reactive properties. However, optimisation of this method is relatively difficult in regard to ensuring minimum NO.sub.x emission since only a few parameters can be adjusted during operation. The amount of fuel burned in the burning compartment depends entirely on the desired degree of calcination of the raw meal.